Solid oxide cells (SOC's) as an example of an electrochemical device generally include cells designed for different applications, such as solid oxide fuel cells (SOFC's), solid oxide electrolysis cells (SOEC's), or membranes. Due to their common basic structure, the same cell may be designed so as to be used in SOFC applications as well as SOEC applications. Since in SOFC's fuel is fed into the cell and converted into power, while in SOEC's power is applied to produce fuel, these cells are referred to as ‘reversible’.
Solid oxide fuel cells (SOFC's) are well known in the art and come in various designs. Typical configurations include an electrolyte layer being sandwiched between two electrodes. During operation, usually at temperatures of about 500° C. to about 1100° C., one electrode is in contact with oxygen or air, while the other electrode is in contact with a fuel gas. Furthermore, a support layer is usually used during the production of the cell to host an electrode layer thereof, whereby said support provides additional mechanical stability of the cell and may also function as, for example, a current collector.
An anode supported cell and a general operational principle in an SOFC mode is shown in FIG. 1. At the cathode, usually comprising lanthanum/strontium manganate (LSM) and yttria stabilized zirconia (YSZ), oxygen ions are formed from the provided oxygen gas, which migrate through the electrolyte layer to combine with the provided hydrogen gas at the anode which comprises YSZ and Ni so as to form water and electrons. The electrons are collected in the anode current collector, which is in FIG. 1 exemplified as a combination of a support for mechanical stability and a current collector, forming a thicker layer.
In advanced electrocatalytic systems, as found in SOC's, the surface chemistry plays a significant role during operation, and the presence of impurities/additives on the respective surfaces has a major influence on the performance and durability of the device.
The manufacture processes for such electrocatalytic systems up to date generally comprise the use of “pure” starting materials so as to avoid any unwanted incorporation of impurities which deteriorate the later performance of the device. “Pure” starting materials are usually commercially available materials having a purity as high as about 99.9%. However, although said starting materials are considered to have a high degree of purity, for purposes of starting materials being used in SOC's they still contain a considerable amount of impurities which deteriorate the later performance of the device especially when present at grain boundaries and reactive electrode sites even at said purity levels of the starting materials.
On a positive side, the presence of said impurities, such as SiO2, Al2O3, alkali- and alkaline earth oxides, and the like, in the oxide starting materials advantageously assist the sintering process by providing a liquid phase. In the finally obtained devices, said the impurities are often found in the form of ultra thin glass films on surfaces, in grain boundaries or at interfaces of the components in the system. The deliberate addition of various sintering aids during the manufacture will also affect the properties of the glass phase. The glass phase may be amorphous, crystalline or a combination thereof.
However, the presence of such impurity phases can result in a decrease in conductivity due to the location in the grain boundaries (GB), a decrease in the catalytic activity due to blocking of the three phase boundaries (TPB) and a delamination of the device due to weakening of the interface, thermal stresses and possible phase changes during operation.
Furthermore, during the manufacture of a solid oxide cell, various additives as an additional source of such impurities may be added intentionally, for example in form of sintering additives. While these sintering additives are improving the layer formation during the manufacture process, it was found that the presence of these additives may nevertheless disadvantageously result in a deteriorated performance of the cell.
Thus, while various additives may be advantageous for improving the manufacturing processes of the cells, they may at the same time disadvantageously represent another source for impurities which may diffuse to grain boundaries and reactive electrode sites during operation and deteriorate the overall cell performance.
US2003/0052392 A1 relates to a device comprising a base which includes a contaminant removing material in form of discrete deposits, which are configured so as to be at least partially exposed to the atmospheric environment of the device. The contaminant removing material is preferably selected from the group consisting of Zr, Ti, Nb, Ta, V and alloys of these metals, and which may further contain Cr, Mn, Fe Co, Ni, Al, Y, La and rare-earth.
U.S. Pat. No. 6,544,665 discloses a thermal barrier coating comprising small amounts of alumina precipitates dispersed throughout the grain boundaries and pores of the coating to oxide getter impurities which would otherwise allow or promote grain sintering and coarsening and pore coarsening.
WO-A-2005/122300 discloses a SOFC cell comprising a metallic support material; an active anode layer consisting of a good hydrocarbon cracking catalyst; an electrolyte layer; an active cathode layer; and a transition layer consisting of preferably a mixture of LSM and a ferrite to the cathode current collector, with means being provided for preventing diffusion between the metallic support and the active anode.
U.S. Pat. No. 6,099,985 discloses a process for precluding coarsening of particles of a first metal in an anode, for use in a solid oxide fuel cell, comprising the steps of:                forming an electrolyte substrate;        preparing a liquid precursor, for a solid solution anode layer containing a first metal and a metal oxide in an amount relative to the first metal effective to substantially preclude coarsening of particles of the first metal in the anode layer, when in use in a solid oxide fuel cell;        decomposing the liquid precursor to form a solid solution containing the first metal and the metal oxide in an amount relative to the first metal effective to substantially preclude coarsening of particles of the first metal in the anode layer, when in use in a solid oxide fuel cell;        converting the solid solution to an anode layer powder;        converting the anode layer powder to an anode suspension material:        placing the anode suspension material onto the electrolyte substrate; and        curing the anode suspension material to form an anode layer disposed upon the electrolyte substrate.        
US-A-2005/0214616 relates to a ceramic-ceramic nanocomposite electrolyte having a heterogeneous structure comprising chemically stabilized zirconia and a nanosize ceramic dopant material selected from Al2O3, TiO2, MgO, BN, and Si3N4.
U.S. Pat. No. 5,419,827 discloses a sintered zirconia ceramic, consisting essentially of:
(a) grains of partially stabilized zirconia consisting essentially of:
(i) 1.5-7.0% by mole of a stabilizer, wherein more than 70% by mole of said stabilizer is yttrium oxide, and
(ii) as the reminder, zirconium oxide and unavoidable impurities; and
(b) grain boundaries including a glass phase containing:
(i) 0.01-2% by weight of MgO based on the sum of said zirconium oxide and said stabilizer;
(ii) 0.1-30% by weight of Al2O3 based on the sum of said zirconium oxide and said stabilizer; and
(iii) 0.3-3% by weight of SiO2 based on the sum of said zirconium and said stabilizer, wherein a critical temperature difference of said sintered zirconia ceramic is larger than or equal to 340° C.
US-A-2004/0166380 discloses a cathode comprising a porous ceramic matrix, and at least an electronically conducting material dispersed at least partially within the pores of the porous ceramic matrix, wherein the porous ceramic matrix includes a plurality of pores having an average pore size of at least about 0.5 μm.
In view of the above, there is a desire to reduce the amount of impurities at the grain boundaries and reactive electrode sites in order to improve the overall performance of the device.